This invention relates to a method and apparatus for magnetically determining the genuineness of documents such as United States currency printed in part at least with magnetizable ink.
Currency validators commonly exploit the fact that certain portions of the currency being validated are printed with magnetic (i.e., magnetizable) ink. Such validators are shown, for example, in U.S. Pat. Nos. 4,114,804, 4,593, 184 and 4,764,725, owned by the assignee herein. Generally, such validators operate by measuring magnetization of ink that is being simultaneously magnetized (the "saturation magnetization") or by measuring the magnetization of ink that has emerged from a magnetizing field (the "remanent magnetization"). The magnetization measured by one of these techniques is typically compared with one or more thresholds to provide an indication of the genuineness of the currency being examined. Both of these techniques, however, rely on the absolute level of measured magnetization, which may vary because of such factors as drive speed, spacing between the head and the note, condition of the note, and other factors unrelated to the genuineness of the note in question.